3GPP Long Term Evolution (LTE) is a standard for mobile phone network technology. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS), and is a technology for realizing high-speed packet-based communication that can reach high data rates on both downlink and uplink channels. As illustrated in FIG. 1, LTE transmissions are sent from base stations 102, such as Node Bs (NBs) and evolved Node Bs (eNBs) in a telecommunication network 106, to mobile stations 104 (e.g., user equipment (UE)). Examples of wireless UE communication devices include mobile telephones, personal digital assistants, electronic readers, portable electronic tablets, personal computers, and laptop computers.
The LTE standard is primarily based on Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, which splits the signal into multiple parallel sub-carriers in frequency, and Single Carrier Frequency Domain Multiple Access (SC-FDMA) in the uplink. A transmit time interval (TTI) is the basic logical unit. A radio resource element (RE) is the smallest addressable location within a TTI, corresponding to a certain time location and a certain frequency location. For instance, as illustrated in FIG. 2, a sub-frame 200 comprised of REs 202 may be transmitted in a TTI in accordance with the LTE standard, and may consist of sub-carriers 204 in the frequency domain. In the time domain, the sub-frame may be divided into a number of OFDM (or SC-FDMA) symbols 208. An OFDM (or SC-FDMA) symbol 208 may include a cyclic prefix 206. Thus, the unit of one sub-carrier and one symbol is a resource unit or element 202.
Wireless communication systems may be deployed in a number of configurations, such as, for example, a Multiple-Input, Multiple-Output (MIMO) radio system. An exemplary MIMO system including a base station 302, such as an eNB, and user equipment 304 is shown in FIG. 3. When a signal is transmitted by the eNB 302 in a downlink, i.e., the link carrying transmissions from the eNB to the UE 304, a sub-frame may be transmitted from multiple antennas 306, 308 and the signal may be received at a UE 304, which has one or more antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. UE 304 may use receiver-diversity signal processing schemes to improve performance.
In an LTE system, transmissions intended for a first user are often overheard by a second, unintended user. The second user may utilize overheard data packets in various ways. For instance, “Completely Stale Transmitter Channel State Information is Still Very Useful,” by M. Maddah-Ali and D. Tse, Allerton Conference, 2010, describes a multi-user downlink MIMO scheme with a mechanism for information exchange between single antenna terminals, where the terminals feedback channel state information (CSI) to the serving base station. The serving base station exploits this CSI to broadcast an additional signal, which each terminal uses to create a virtual diversity receiver (VDR). This type of information exchange may be referred to as “stale feedback,” since the channel may have changed significantly by the time the base station transmits the extra signal. In this scheme, a mobile device that receives signals on only a single antenna may still take advantage of simple receive-diversity processing techniques. Similarly, “Multi-User ARQ,” by Peter Larsson and Nicklas Johansson, IEEE VTC Spring, 2006, discusses an Automated Repeat request (ARQ) control scheme that exploits the fact that users frequently overhear each other's information.
Absent from the presently known schemes is an explicit pilot structure that can be effectively used to facilitate the estimation of channel parameters at the receivers, including true channel taps, as well as estimations of the virtual channels created by the VDR scheme. The lack of pilot structure renders these schemes incomplete and unusable in practice.
Accordingly, there is a need for a pilot structure to support a virtual diversity receiver scheme in an LTE communication network in order to improve transmission quality.